3-Deoxy-D-arabino-heptulosonic acid 7-phosphate (DAHP) synthase (EC 4.1.2.15), the first enzyme of the multibranched pathway that leads to the synthesis of the three aromatic amino acids phenylalanine, tyrosine and tryptophan and to the synthesis of several vitamins and the iron-chelating compounds enterochelin, appears in Escherichia coli in three isoenzymic forms, each feedback-inhibited by one of the aromatic amino acids. We have purified to homogeneity two of the isoenzymes, and we have started the physico-chemical characterization of these proteins. Amino acid sequence analysis of the typrosine-sensitive isoenzyme will be completed, the crystallization of the protein will be attempted. Isolation of plasmids carrying structural genes for the isoenzymes will be continued. Monospecific antibodies against the tyrosine-sensitive isoenzyme will be used to study biosynthesis and degradation of the enzyme in vivo. Immunological similarities between DAHP synthases from different organisms will be delineated. The third isoenzyme from Escherichia coli will be purified to homogeneity.